This invention relates to a stepwise process for forming printed circuits which includes a step of selectively etching a first metal layer while retaining a second metal layer which forms the printed circuit intact.
Prior to the present invention, printed circuits have been formed by additive, semi-additive or subtractive processes. Most commonly existing processes are subtractive processes based upon starting with a copper foil laminated to one or both sides of a substrate, which is either a rigid sheet or a flexible film material. In common processes, electrical connections are provided between foil layers on two sides of a substrate (board) or between the foil layers of multilayered boards. After electroplating copper onto circuit surfaces to a desired thickness, the portion of the copper foil laminate which does not form the printed circuit is removed by contacting it directly with a liquid etching composition which dissolves the excess copper. In this step, the portion of the metal laminate forming the circuit must be protected by a resistant coating that is usually either a polymeric composition or a thin plated layer of tin and lead alloy.
Using liquid chemical etchents for dissolving this copper has two undesirable results. It causes some etching of copper that comprises the desired circuit pattern along and under the edges of the protective mask layer. This limits the fineness of lines that can be formed and thus the number and spacing of lines on a given area of a board.
Electroless copper plating and chemical etching of copper also results in the formation of chelated solutions of copper that must be treated and handled as hazardous wastes. In order to effect safe disposal of these metal containing solutions, they must be treated to remove the metal content prior to disposal. The required wastewater treatment processes are costly and time consuming since they require the use of reagents and capital equipment as well as skilled personnel trained in the handling of such solutions and reagents.
It has been proposed in U.S. Pat. No. 4,659,426 to utilize a plasma containing a halogen based gas and either carbon monoxide or carbon dioxide in order to etch fine lines in a thin metal coating on a substrate. A variety of metals and metallic compounds can be etched by this process which is described as a method for fabricating an integrated circuit such as on a chip of silicon. This process is undesirable for fabricating a printed circuit since it requires an expensive apparatus for producing a plasma capable of etching fine lines and the high plasma temperatures would exceed the normal limits of common printed circuit substrate materials.
It is also known by the Mond Process that nickel reacts with carbon monoxide at about 80.degree. C. to form gaseous nickel carbonyl which dissociates to form metallic nickel particles and carbon monoxide gas at about 200.degree. centigrade.
Accordingly, it would be desirable to provide a process for producing printed circuits which eliminates using electroless copper plating and liquid chemical etching of metals. Additionally, it would be desirable to provide a process for recovering etched metal for recycling and reuse. Furthermore, it would be desirable to provide such a process which eliminates the need for much of the expensive equipment used for forming printed circuits and for treating the wastes produced by existing methods.